Shiqian Hu

Experimental study of thermal rectification in suspended monolayer graphene

Thermal rectification is a fundamental phenomenon for active heat flow control. Significant thermal rectification is expected to exist in the asymmetric nanostructures, such as nanowires and thin films. As a one-atom-thick membrane, graphene has attracted much attention for realizing thermal rectification as shown by many molecular dynamics simulations. Here, we experimentally demonstrate thermal rectification in various asymmetric monolayer graphene nanostructures. A large thermal rectification factor of 26% is achieved in a defect-engineered monolayer graphene with nanopores on one side. A thermal rectification factor of 10% is achieved in a pristine monolayer graphene with nanoparticles deposited on one side or with a tapered width. The results indicate that the monolayer graphene has great potential to be used for designing high-performance thermal rectifiers for heat flow control and energy harvesting.

A Series Circuit of Thermal Rectifiers: An Effective Way to Enhance Rectification Ratio

A novel approachis proposed to enhance the thermal rectification ratio, namely, arranging two thermal rectifiers in series. Through theoretical analysis and molecular dynamics simulations on graphene/phononic crystal structures, the results show that the series thermal rectifiers enhance thermal rectification ratio significantly, compared to a single rectifier. Meanwhile, the results of theoretical prediction match well with simulation results.

Manipulating the temperature dependence of the thermal conductivity of graphene phononic crystal.

By using non-equilibrium molecular dynamics simulations, modulating the temperature dependence of thermal conductivity of graphene phononic crystals (GPnCs) is investigated. It is found that the temperature dependence of thermal conductivity of GPnCs follows ∼T (-α) behavior. The power exponents (α) can be efficiently tuned by changing the characteristic size of GPnCs. The phonon participation ratio spectra and dispersion relation reveal that the long-range phonon modes are more affected in GPnCs with larger holes (L 0). Our results suggest that constructing GPnCs is an effective method to manipulate the temperature dependence of thermal conductivity of graphene, which would be beneficial for developing GPnC-based thermal management and signal processing devices.

Nanoscale Graphene Disk: A Natural Functionally Graded Material-How is Fourier's Law Violated along Radius Direction of 2D Disk.

In this Paper, we investigate numerically and analytically the thermal conductivity of nanoscale graphene disks (NGDs), and discussed the possibility to realize functionally graded material (FGM) with only one material, NGDs. Different from previous studies on divergence/non-diffusive of thermal conductivity in nano-structures with different size, we found a novel non-homogeneous (graded) thermal conductivity along the radius direction in a single nano-disk structure. We found that, instead of a constant value, the NGD has a graded thermal conductivity along the radius direction. That is, Fourier’s law of heat conduction is not valid in two dimensional graphene disk structures Moreover, we show the dependent of NGDs’ thermal conductivity on radius and temperature. Our study might inspire experimentalists to develop NGD based versatile FGMs, improve understanding of the heat removal of hot spots on chips, and enhance thermoelectric energy conversion efficiency by two dimensional disk with a graded thermal conductivity.

Randomness-Induced Phonon Localization in Graphene Heat Conduction

Through nonequilibrium molecular dynamics simulations, we report the direct numerical evidence of the coherent phonons participating in thermal transport at room temperature in graphene phononic crystal (GPnC) structure and evaluate their contribution to thermal conductivity based on the two-phonon model. With decreasing period length in GPnC, the transition from the incoherent to coherent phonon transport is clearly observed. When a random perturbation to the positions of holes is introduced in a graphene sheet, the phonon wave-packet simulation reveals the presence of notable localization of coherent phonons, leading to the significant reduction of thermal conductivity and suppressed length dependence. Finally, the effects of period length and temperature on the coherent phonon contribution to thermal conductivity are also discussed. Our work establishes a deep understanding of the coherent phonons transport behavior in periodic phononic structures, which provides effective guidance for engineering thermal transport based on a new path via phonon localization.